The present invention has found wide application in semiconductor laser diodes presently used in a variety of information handling systems. Their compact size and manufacturing process have some unique advantages, particularly in making them compatible with associated electronic circuitry within the same substrate. Laser diodes of this type have successfully been used in data communication, optical storage and laser beam printing, and have been optimized in applications which advantageously use their wavelength, optical power, high speed modulation characteristics and beam quality.
The invention herein described is of interest to short wavelength lasers, and more particularly to laser printers and optical disk systems requiring narrow beams at high optical power. Very attractive for such applications are (Al)GaInP laser diodes operating in the 0.6 .mu.m wavelength range. These short wavelength materials, however, have characteristics that are noticeably different from (Al)GaAs systems and which potentially can introduce additional problems. Consequently, concepts and designs adequate for (Al)GaAs devices are not generally suited for visible light lasers.
The present invention provides a solution to two key requirements: a strong current and carrier confinement to obtain low threshold, highly efficient laser diodes. Additionally, it provides protection against catastrophic optical mirror damage occurring as a result of heat developing at the mirror facets, which limits the high output power operation in long lifetime devices.
Substantial improvements in carrier and current confinement have been achieved with the so-called "buried structures" wherein the active layer of the laser is embedded in a wider bandgap material in a direction parallel to the waveguide that shapes the laser cavity. However, known structures require complex fabrication processes. Most commonly used is a process that starts with an etch step to obtain a mesa including the GaInP active layer. The mesa is then buried using a sequence of epitaxial regrowth steps. This technology is not only accompanied by a strong loss in device yield, but it can also cause severe device reliability problems.
An example of a buried structure and its fabrication process is disclosed in the European Patent Application 0 348 540 "Process for the selective Growth of GaAs". Here, a patterned substrate with horizontal and inclined surface sections of different crystal orientation is used to provide a selective growth of an active layer on horizontal surfaces whereas the embedding wider bandgap cladding layers grow on both, the horizontal and the inclined surfaces. Like other methods that rely on a crystal orientation that depends on no or reduced-growth effects, this process is problematic in that, during the epitaxy process, no or reduced-growth unprotected facets may be contaminated, thereby affecting the crystal structure.
Efforts have also been invested in the design of laser structures having non-absorbing mirrors (NAM) to avoid excessive heating by non-radiative carrier recombination near the mirror facets. Here, the general approach is to terminate the active layer with a so-called "window structure" consisting of a transparent, wider bandgap material, thereby substantially reducing the number of carrier pairs near the facet. NAM laser structures of this nature have been disclosed in several publications and patent applications.
Article, "Non-Absorbing Laser Mirrors", published in the IBM Technical Disclosure Bulletin, Vol. 31, No. 2, July, 1988, p. 240, describes an (Al)GaAs laser consisting of a layered structure comprising an active GaAs quantum well layer embedded between cladding layers. The layers are grown on the structured surface of a GaAs substrate with the center portion on a (100)-plane, and both mirror regions on (411)-planes. Since the GaAs growth speed is markedly slower on (411)-surfaces, the width of the active quantum well layer is reduced near the mirror regions. This results in an increased effective bandgap.
European Patent Application 0 332 723 "High-Power Semiconductor Diode Laser", shows a laser structure formed on a patterned substrate having planar mesa and groove-sections with inclined transition zones in between. The layered structure comprises an active and a passive waveguide. The gain segment of the active waveguide is aligned with the non-absorbing section of the passive waveguide. Light generated in the gain section is coupled to the non-absorbing section through an inclined cladding layer where it is fully guided as it propagates towards the mirror facets.
Article, "Vertically Emitting Laser with integrated NAM deflector", published in the IBM Technical Disclosure Bulletin, Vol. 32, No. 3B, August, 1989, pp. 498-499, describes other forms of non-absorbing mirror or deflector structures. In one particular instance, the active layer, made of quantum wells, is disordered by ion implantation or diffusion, thereby rising the bandgap of the active layer.
Article, "Novel Window-Structure AlGaInP visible LDs with non-absorbing Facets formed by disordering of natural Superlattice in GaInP Active Layer", by Y. Ueno et al, published by the 12th IEEE International Semiconductor Laser Conference, Davos/Switzerland, September, 1990, pp. 30-31, which describes a visible light AlGaInP laser diode with a higher bandgap (E.sub.g) crystal incorporated at the mirror facets. This is accomplished by selective disordering of the GaInP active layer with diffused impurity (Zn) near the facets where (E.sub.g) increases. A marked increase in maximum light output power is detected.
Article, "Window Structure InGaAlP Visible Light Laser Diodes by Self-Selective Diffusion-Induced Disordering" by K. Itaya et al, published in the same Conference Digest as the previous reference, pp. 36-37, Zn-diffusion-induced disordering is used to create a wider bandgap window region for an InGaAlP laser structure. The diffusion is obtained below an n-cap layer selectively left in the mirror region.
These known non-absorbing mirror concepts require rather complex structures and fabrication processes and run counter to the demand for higher yield and "easy-to-make" non-absorbing mirror laser diodes.
The present invention discloses a new design of laser devices in which a strong current and carrier confinement that includes non-absorbing mirror waveguides can be realized in a single epitaxial step. This is achieved through local bandgap variation by simultaneous in-situ growth of ordered and disordered phase semiconductor materials of the Group III on a structured substrate.
The present invention is based on the phenomenon that certain semiconductor materials, especially (Al)GaInP, which have become very important for visible light laser, have been found to exist in several phases, i.e., ordered and disordered, which differ in the atomic arrangement of group III species. They have also been known to display a bandgap energy E.sub.g that increases when changing from the ordered to the disordered phase, and to grow under standard growth conditions in the ordered phase on a standard substrate orientation (or on a slightly misoriented surface). The disordered phase is, on the other hand, obtained in case of stronger misoriented substrates.
These material characteristics have been investigated and reported on in the following publications:
"Evidence for the Existence of an ordered State in GaInP grown by Metal Organic Vapor Phase Epitaxy and its Relation to Band-Gap Energy", by A. Gomyo et al, published in Appl. Phys. Lett. 50(11), Mar. 16, 1987, pp. 673-675. It demonstrates that epitaxial layers can assume states of different bandgap energy (E.sub.g) corresponding to an ordered and random (Ga,In) distribution on column III sublattices depending on III/V ratios and growth temperatures.
"Dependence of Photoluminescence Peak Energy of Metal Organic Vapor Phase Epitaxy (MOVPE)-grown AlGaInP on Substrate Orientation", by S. Minagawa et al, published in Electronic Letters, Jun. 8, 1989, Vol. 25, No. 12, pp. 758-759, which describes measurements of GaInP and AlGaInP materials grown by Metal Organic Vapor Phase Epitaxy. It has been found that the bandgap increases as the substrate misorientation with respect to the (100)-plane increases and saturates at high inclination angles due to a complete transformation into a random alloy.
These papers describe the material properties upon which the present invention is based but not suggestion is made on the use of structured substrates to selectively achieve regions of distinct bandgaps, nor do they propose the application of the investigated phenomena to achieve buried heterostructures and/or non-absorbing mirror facets in diode laser technology.